The MAGPI Survey: forward modelled gas-phase metallicity gradients in galaxies at $z\sim 0.3$

TL;DR

This study measures seeing-deconvolved gas-phase metallicity gradients in 70 star-forming galaxies at $z\sim0.3$ using Blobby3D forward modelling to correct for beam smearing and capture flux substructure. Metallicity is inferred via N2O2 (primary) and N2H$\alpha$ diagnostics, with cross-validation and extinction corrections incorporated; gradients are extracted in elliptical annuli and normalized by $R_e$. The median gradient is $-0.013^{+0.059}_{-0.033}$ dex/kpc, with $32.9\%$ of galaxies showing significant negative gradients, $10\%$ positive, and $57.1\%$ flat; $\nabla\mathrm{[O/H]}$ correlates positively with $\sigma_{gas}$ and $\Sigma_{SFR}$, and negatively with $R_e$, while partial correlations emphasize $\sigma_{gas}$ as the strongest driver. The results support a picture where stellar feedback, gas transport, and accretion govern metallicity distributions, with smaller galaxies more susceptible to mixing and dilution, and show only mild redshift evolution of gradients, consistent with prior measurements and simulations when beam-smearing is accounted for.

Abstract

We measure the seeing-deconvolved gas-phase metallicity gradients of 70 star-forming galaxies at $z\sim 0.3$ from the MAGPI survey and investigate their relationship with galaxy properties to understand the mechanisms that influence the distribution of metals and shape the evolution of the galaxies. We use a Bayesian modelling technique, Blobby3D, which accounts for seeing effects (beam smearing) and can model the substructures of the flux distribution. The median metallicity gradient of our sample is $\nabla \mathrm{[O/H]}=-0.013^{+0.059}_{-0.033}$ dex/kpc. Among the galaxies in our sample, 32.9% have negative metallicity gradients (2$σ$ significance), 10.0% have positive gradients and 57.1% have flat gradients. The $\nabla \mathrm{[O/H]}$-$M_*$ relation of the MAGPI galaxies generally agrees with theoretical predictions, where a combination of stellar feedback, gas transport, and accretion shapes the metallicity profile, with the dominant processes varying with galaxy mass. We find a positive correlation between $\nabla \mathrm{[O/H]}$ and gas velocity dispersion ($r=0.36$), indicating that stronger gas turbulence is associated with flatter or inverted metallicity gradients, likely due to enhanced gas mixing. Additionally, smaller galaxies tend to have flatter or positive gradients, suggesting that metal dilution by gas accretion or removal via feedback-driven winds may outweigh metal enrichment in small galaxies.

Figures (13)

• Figure 1: The distribution of H$\alpha$-based SFR and $M_*$ for all star-forming galaxies and galaxies in our final sample. The solid line represents the SFMS for galaxies at $z\sim 0.35$, with the dashed lines representing the root-mean-square error of SFMS Mun:2024.
• Figure 2: The metallicity and metallicity gradient measurement for an example galaxy (MAGPIID 1203195161). The top-left panel shows a colour composite image of the galaxy, constructed from the $i$-, $r$-, and $g$-band data. The first row shows the metallicity measured using the N2O2 diagnostic and the second row uses the N2H$\alpha$ diagnostic. The middle panels show the seeing-deconvolved metallicity map. The right panels demonstrate the metallicity of each spaxel as a function of radius (grey) and the average metallicity in each 0.5 PSF$_\mathrm{FWHM}$ bin for three different weight methods. We overplot the linear best fits for each method. The horizontal lines in the right panels denote the valid metallicity range for each diagnostic Pettini:2004Kewley:2019. The green vertical lines denote 1 $R_\mathrm{e}$ for this galaxy. We show the value of the metallicity gradient and central metallicity (the inverse variance weighted method) for each plot.
• Figure 3: The metallicity and metallicity gradient measurement for an example galaxy (MAGPIID 1508217276). The same as Figure \ref{['fig:metal_example']}.
• Figure 4: The left panel shows the distribution of the radius of the most extended radial bin ($r_\mathrm{max}$) relative to PSF$_\mathrm{FWHM}$. The right panel shows the distribution of $r_\mathrm{max}$ with respect to $R_\mathrm{e}$. Most of the galaxies are moderately resolved, extending out to a median of $R\sim2\,R_\mathrm{e}$.
• Figure 5: The N2O2 metallicity gradients (top two rows in units of dex/kpc, bottom two rows in units of dex/$R_\mathrm{e}$) as a function of a variety of galaxy properties, including stellar mass ($M_*$), $R_\textup{e}$, a proxy for the gravitational potential ($M_*/R_\mathrm{e}$), $\Sigma_{M_*}$, SFR, $\Sigma_\textup{SFR}$, $\sigma_\textup{gas}$ and $v_{\phi}/\sigma_\mathrm{gas}$. We show the Pearson correlation coefficient, $r$, and $p$-value on each panel. The horizontal dashed lines denote the flat gradient. The $\nabla \mathrm{[O/H]}$ of MAGPI galaxies has a moderately significant positive correlation with $\sigma_\textup{gas}$ and with $\Sigma_\textup{SFR}$ and negative correlation with $R_\mathrm{e}$.